Implant and articular prosthesis comprising said implant

ABSTRACT

An implant for an articular prosthesis comprising a first part ( 3 ) made of a polymer material defining a cavity ( 4 ) in which a second part ( 7 ) made of a ceramic material is accommodated, defining a cavity ( 8 ) which is essentially hemispherical in shape and which receives the head ( 2 ) of a second implant also comprising a rod ( 1 ), characterized in that the edge ( 10 ) of the second part ( 7 ) is buried in the first part ( 3 ) which is produced by duplicate moulding on the second part ( 7 ), also characterized in that the edge ( 9 ) of the first part ( 3 ) is provided with a chamfer ( 11 ′) against which the rod ( 1 ) of the second implant is thrust. An articular prosthesis comprising a first implant and a second implant comprising a rod ( 1 ) which is terminated by substantially spherical hed ( 2 ) which turns in a cavity ( 8 ) whose shape corresponds to that of the first implant, characterized in that the first implant is of the previous type.

The invention concerns the field of articular prostheses such as hip orshoulder prostheses.

It is customary to use hip prostheses formed, on the one hand, by ametal rod and a femoral head made of ceramic and of substantiallyspherical shape which the surgeon uses to replace the upper portion ofthe femur of the patient, and, on the other hand, by an acetabularimplant which is intended to receive said femoral neck and which thesurgeon implants, for example by sealing, in the patient's pelvis as areplacement for the natural acetabulum.

In a known preferred example of such a prosthesis, the acetabularimplant is composed of a first part made of a polymer material, forexample polyethylene, the general shape of which is that of a cup, andof a second part which is made of a ceramic material, for examplealuminum, and which lines the inside of the first part. This second partdefines a seat whose shape corresponds to that of the femoral head ofthe prosthesis. The inner edge of this seat is chamfered in such a waythat it does not have a sharp angle and in such a way as to permit agiven maximum inclination of the metal rod during the movements of thepatient's thigh.

A problem posed by this configuration is that, when the metal rodassumes its maximum inclination, it strikes against the inner chamferededge of the seat formed in the ceramic part. This represents a source ofwear of the ceramic, leading to undesirable dispersion of particles inthe region of the articulation, or even fracturing of the ceramic partaffected by the impacts. Moreover, the rod then bears on the edge of theseat, and, if the relative movement of the thigh and of the pelvis seeksto continue, the normal geometry of the acetabular cups means there is asubstantial risk of the femoral head escaping from its seat as a resultof a “cam effect”, leading to luxation of the hip.

The object of the invention is to propose a novel acetabular implantdesign allowing the aforementioned problems to be remedied.

To this end, the subject of the invention is an implant for an articularprosthesis, of the type comprising a first part made of a polymermaterial and defining a cavity in which a second part made of a ceramicmaterial is fitted, said second part defining a cavity of substantiallyhemispherical shape intended to receive a head of a second implant whichalso includes a rod, characterized in that the edge of said second partis recessed in the first part which is produced by duplicate molding onthe second part, and in that the edge of said first part has a chamferagainst which the rod of the second implant is intended to be able toabut.

Said first part preferably includes, in line with the upper limit of thecavity of the second part, a wall portion intended to form an abutmentfor the head during movements that are liable to cause luxation of thearticulation.

The invention also relates to an articular prosthesis, of the typecomprising a first implant and a second implant with a rod terminated bya head of substantially spherical shape which turns in a cavity ofcorresponding shape in the first implant, characterized in that saidfirst implant is of the above type.

This articular prosthesis can in particular constitute a hip prosthesisor a shoulder prosthesis.

As will be understood, the invention consists in performing duplicatemolding of the ceramic part of the first implant by a polymer materialcovering all of the upper edge of said ceramic part. The chamferlimiting the amplitude of the inclinations of the rod of the secondimplant is formed on the duplicate mold, such that the rod, which isgenerally made of metal, never comes into contact with a ceramic part.The only contacts that may exist in the articular prosthesis accordingto the invention are therefore ceramic-ceramic contacts andmetal-polymer contacts. There is therefore no longer any metal-ceramiccontact capable of causing substantial wear or fracturing of theceramic. Moreover, the invention can lead to a modification of thegeometry of the first implant, making luxation of the articulation moredifficult.

In the preferred example of application of the invention to a hipprosthesis, the first implant is an acetabular implant, and the secondimplant replaces the terminal portion of the femur of the patient.

The invention will be better understood from reading the followingdescription in which reference is made to the attached figures, inwhich:

FIG. 1 is a schematic cross section through a hip prosthesis accordingto the prior art,

FIG. 2 is a schematic cross section through a hip prosthesis accordingto the invention, derived from the prosthesis in FIG. 1.

The hip prosthesis according to the prior art as shown in FIG. 1includes the following elements:

an implant formed by a metallic rod 1 made of a biocompatible metal(stainless steel or titanium, for example) having, at its end, a femoralhead 2 which is made of a ceramic such as aluminum and which is of asubstantially spherical shape, this assembly of rod 1 and head 2 beingintended to replace the terminal portion of the femur of the patient;

an acetabular implant which receives the femoral head 2, is intended toreplace the acetabulum of the femoral articulation of the patient, andis composed of two parts:

-   -   a first part 3 made of a polymer material such as polyethylene,        with an approximately hemispherical outer shape and provided        with a cavity 4 defining a substantially spherical surface; this        first part is intended to be implanted in the patient's pelvis,        for example by sealing with the aid of a cement; to facilitate        fixation in the pelvis, the outer surface 5 of the first part 3        includes various grooves and recesses 6;    -   a second part 7 made of a ceramic such as aluminum, lining the        cavity 4 of the first part, and defining the seat 8 for the        femoral head 2; for this purpose, this seat 8 has a spherical        surface corresponding to the geometry of the femoral head 2.

It will be noted that the edges 9, 10 of the first 3 and second 7 partsdirected toward the outside of the articulation are aligned in such away as to define a single edge of the acetabular implant. The edge 10 ofthe second part 7 has a chamfer 11 on which, as is shown, the metal rod1 bears at the maximum permitted clearance of the femur in thearticulation (according to the arrow 12). As has been stated, thiscontact between metal and ceramic generates wear of the ceramic, even arisk of fracturing, in the area of the chamfer 11. Moreover, the chamfer11 thus constitutes a bearing zone for the rod 1. If the movement of therod 1 seeks to continue, this risks causing the head 2 to escape fromthe seat 8 in the direction of the arrows 13, in other words luxation ofthe patient's hip.

In the example of a hip prosthesis according to the invention shown inFIG. 2, the implant formed by the assembly of rod 1 and head 2 is oncemore present. The acetabular implant once again is made up of a firstpart 3 made of a polymer and with a general shape similar to that inFIG. 1, likewise with ribs and recesses 6 on its outer surface 5, and ofa first part 7 made of ceramic and defining the seat 8 for the femoralhead 2. The main difference from the configuration in the prior artshown in FIG. 1 its that here, according to the invention, the upperedge 9 of the first part 3 alone constitutes the upper edge of theacetabular implant. The upper edge 10 of the second part 7 is thusrecessed inside the first part 3 which is produced by duplicate moldingon the second part 7. The main consequence is that the chamfer 11′,which limits the clearance of the rod 1 and thus performs the samefunction as the chamfer 11 of the prior art, is no longer formed on thesecond part 7 made of ceramic, but instead on the upper edge 9 of thefirst part 3 made of polymer. The contacts between the rod 1 and thechamfer 11′ are therefore metal-polymer contacts which do not riskcausing release of particle or pieces of ceramic into the patient'sbody.

The production of the first part 3 by duplicate molding on the secondpart 7 permits excellent complementarity between the two parts, andtherefore an excellent hold of the second part 7 by the first part 3.This is crucial since the first part is fixed directly in the patient'spelvis.

Another advantage of the invention is that, as can be seen from FIGS. 1and 2, while keeping similar dimensions for the main parts of theprosthesis (total size of the assembly, thickness of the various parts)it is possible to give the chamfer 11′ a greater inclination withrespect to the vertical than in the prior art. The possible angle ofclearance of the rod 1 is thus increased. This, together with thefarther distance of the center of curvature of the movement of the rodfrom the edge of the seat for the head 2 when it bears on the chamfer11′, makes it possible to reduce the risk of a cam effect beingproduced, as has been described above. Moreover, it is possible to formon the first part 3, in line with the upper limit of the cavity 8 of thesecond part 7, a wall portion 14 which continues said cavity 8 and whichis intended to form an abutment for the head 2 when the rod 1 seeks tocontinue its movement after coming into abutment against the chamfer11′. Luxation thus becomes more difficult.

It will be noted that the thickness of the polymer covering the edge 10of the second part 7 must be at least 0.5 mm. It can of course beappreciably greater.

By way of example, a hip prosthesis according to the invention can havethe following dimensions:

external dimension of the first part 3 made of polymer: 30×50 mm;

thickness of the first part 3 made of polymer: 6 mm;

thickness of the second part 7 made of ceramic: 4 mm;

radius of curvature of the seat 8 for the head 2: 14 mm;

inclination of the chamfer 11′ with respect to the vertical: 60°

height of the vertical wall portion 14: 4 mm.

The invention has been described and depicted in its application to ahip prosthesis. However, it can be applied to other types of articularprostheses having an analogous function, for example shoulderprostheses.

1. An implant for an articular prosthesis, of the type comprising afirst part (3) made of a polymer material and defining a cavity (4) inwhich a second part (7) made of a ceramic material is fitted, saidsecond part (7) defining a cavity (8) of substantially hemisphericalshape intended to receive a head (2) of a second implant which alsoincludes a rod (1), characterized in that the edge (10) of said secondpart (7) is recessed in the first part (3) which is produced byduplicate molding on the second part (7), and in that the edge (9) ofsaid first part (3) has a chamfer (11′) against which the rod (1) of thesecond implant is intended to be able to abut.
 2. The implant as claimedin claim 1, characterized in that said first part (3) comprises, in linewith the upper limit of the cavity (8) of the second part (7), a wallportion (14) intended to form an abutment for the head (2) duringmovements that are liable to cause luxation of the articulation.
 3. Anarticular prosthesis, of the type comprising a first implant and asecond implant with a rod (1) terminated by a head (2) of substantiallyspherical shape which turns in a cavity (8) of corresponding shape inthe first implant, characterized in that said first implant is of thetype as claimed in claim
 1. 4. The articular prosthesis as claimed inclaim 3, characterized in that it constitutes a hip prosthesis, saidfirst implant being an acetabular implant and said second implantreplacing the terminal portion of the femur of the patient.
 5. Thearticular prosthesis as claimed in claim 3, characterized in that itconstitutes a shoulder prosthesis.
 6. An articular prosthesis, of thetype comprising a first implant and a second implant with a rod (1)terminated by a head (2) of substantially spherical shape which turns ina cavity (8) of corresponding shape in the first implant, characterizedin that said first implant is of the type as claimed in claim 2.